Topological Defect-Guided Regular Stacking of Focal Conic Domains in Hybrid-Aligned Smectic Liquid Crystal Shells

Junghyun Noh,Jan P F Lagerwall

doi:10.3390/cryst11080913

Abstract

We study liquid crystal (LC) shells in hybrid configuration (director tangential to the inside but normal to the outside) as they slowly undergo a transition from a nematic (N) to a smectic-A (SmA) phase. Every shell has two antipodal +1 topological defects, at the thinnest and thickest points, respectively. On cooling from N to SmA, the symmetry axis connecting the defects gradually reorients from along gravity to perpendicular to it, reversibly and continuously, if the LC and aqueous phase are density matched at the N-SmA transition. This suggests reduced density near the defects—reflecting a local reduction in order—under the strong confinement with antagonistic boundary conditions. In the SmA phase, a regular array of focal conic domains (FCDs) develops, templated in position and orientation by the +1 defect at the thinnest point. Around this defect, a single complete toroidal FCD always develops, surrounded by incomplete FCDs. In contrast to similar FCD arrangements on flat aqueous interfaces, this is a stable situation, since the two +1 defects are required by the spherical topology. Our results demonstrate how the topological defects of LC shells can be used to template complex self-organized structures. With a suitable adaption of the LC chemistry, shells might serve as a basis for producing solid particles with complex yet highly regular morphologies.

Highlights

The liquid crystal (LC) nematic (N) to smectic-A (SmA) transition under confinement between boundaries imposing orthogonal anchoring of the director n constitutes a very rich system for soft matter self-organization studies, since this configuration induces a frustrated situation in the SmA phase that does not exist in the
We speculate that the reason for the surprising stable equilibrium configuration is the free energy contribution of the very strong director field bend at the thinnest point of the shell, which may be enough to turn the LC to locally nematic at that point
Hybrid-aligned LC shells undergoing a transition from nematic to smectic-A phases constitute a highly interesting platform for studying the complex self-organization taking place in response to the spherical confinement

Summary

Introduction

The latter adapts to the boundary conditions by adopting a continuous bend in the director field, n(r), from one boundary to the other, but the SmA phase cannot accommodate bend due to its one-dimensional positional order with molecules arranging in layers that orient perpendicularly to n [1,2]. This inability is often reflected in the N phase upon approaching the N-SmA transition, as a divergence of the elastic constant K3 , describing the energy density of bend deformation. The effect is recognized in polarizing microscopy of a hybrid-aligned LC undergoing the

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Discussion

Conclusion